A Lose Lose Situation

I'd planned on visiting my fiancé in March next year to go to a convention and to see her during her birthday, but my mum wants me to go with her to Las Vegas in May. She said it'll be her last trip to the US because of her arthritis and she won't be able to hire a car over there after she reaches 65. Unfortunately I can't afford both trips, even though I really want to do both. My Fiance is hurting because I keep having to postpone seeing her. The first time was last January when I didn't feel comfortable travelling by myself (I've overcome that fear now), the second was because I lost my passport and the third time was because of my mum saying horrible things and creating problems out of it...It's getting to the point where my fiance is starting to question whether or not to wait for me. She said she doesn't feel like she's my priority or important to me and that I don't want to come and see her even though I do. If I don't visit her then I risk losing her because I've already lost some of her trust when it comes to me saying I'll visit her, so if I go to Vegas with my mum then I'll feel horrible the entire time because it'll hurt her a lot and I'll risk losing her. On the other hand, if I visit my fiancé and don't go with my mum, I'll regret it because it's our thing that we've been planning for a long time and it'll be her last holiday to the US. I also don't know if my mum knows about our relationship because she keeps suggesting that I get a boyfriend but at the same time she's been screenshotting romantic posts that my fiance and I make for each other so I don't know. This whole situation is making me feel like I'm going crazy...I feel like I'm being pulled in every direction and there's no answer to it. I feel like it's a lose lose situation and I really don't know what to do for the best